This invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting pigments without the use of chromium in the hexavalent oxidation state or lead in the tetravalent oxidation state. More particularly, this invention relates to non-toxic, corrosion-inhibiting pigments based on trivalent and tetravalent manganese and methods of making and using the same.
Inhibiting the initiation, growth, and extent of corrosion is a significant part of component and systems design for the successful long-term use of metal objects. Uniform physical performance and safety margins of a part, a component, or an entire system can be compromised by corrosion. Aluminum, zinc, iron, magnesium, titanium and their alloys tend to corrode rapidly in the presence of water due to their low oxidation-reduction (redox) potentials. The high strength 2000 and 7000 series of aluminum alloys are used extensively in aircraft and are very sensitive to corrosive attack. Materials such as steels and carbon fibers with higher redox potentials will form a galvanic couple in water and promote corrosive attack when located near light metal alloys such as aluminum.
A bare metal surface or one that has been conversion coated, phosphated, sealed, rinsed, or otherwise treated will be protected by the application of a primer paint with a corrosion inhibiting pigment. As used herein, the term “pigment” means chemically active compounds with the ability to inhibit corrosion at a distance, rather than simple colorants or opacifiers. Oxidative compounds that are effective as corrosion inhibitors tend to be highly colored and/or opaque. An effective corrosion inhibiting pigment has throwing power and can protect exposed base metal in a scratch or flaw by oxidizing and passivating it at a distance during aqueous corrosion when dispersed in a suitable carrier phase. These compounds are usually solids or liquids that are typically dispersed in a liquid carrier or binder system such as a paint or wash. These compounds may also be used to help inhibit corrosion without a significant liquid carrier using an integral binder and/or a low-volatile application method. Barrier layer formers such as sol-gel coatings or polymeric films are also used, but they tend to have no inherent oxidizing character and no appreciable throwing power and fail to protect the metal surface when the film is breached.
Pigments that contain hexavalent chromium (CrVI) compounds are the de facto standard for high-performance corrosion inhibiting paints and coatings for metal protection and are a typical corrosion inhibitor used to protect aluminum, zinc, magnesium, iron, titanium, copper and their alloys. Zinc (C.I. Pigment Yellow 36) and strontium (C.I. Pigment Yellow 32) chromate pigments are typically used, although higher-solubility calcium and magnesium chromates have also seen some limited use as pigments. The coating vehicles of these pigments include alkyd-type primers, acrylic primers, and elastomeric sealants, among others. Some transition metal chromate pigments (e.g., complexed with copper, iron, manganese, or cobalt) and organic chromate pigments (e.g., bound with nitrogenous compounds such as guanidinium) have been used in protective coating systems. Barium or lead chromates have also been used as corrosion inhibitors, as well as colorants. Variations in chromate speciation (i.e., what the chromate ions are bound to) will result in significant differences in protection when used as corrosion-inhibiting pigments, due to differences in chromate solubility.
A clear correlation between performance and solubility of chromate pigments has been shown. However, oxidizing chromates can be dangerous to use as corrosion inhibitors if they are not delivered in sufficient quantity in a timely manner to the location of a coating breach. The chromate composition was far more important to the corrosion inhibiting performance of the primer film than the organic coating composition.
A principle use of zinc and strontium chromate pigments is in wash- or etch-primer formulations for aluminum protection. Wash- or etch-primers, which have been used since the 1940s, represent one of the harshest application conditions for chromate pigments. Wash-primers are applied to metal surfaces under acidic conditions where the primer is cured as a corrosion inhibiting film. Chromate pigment powders dispersed in an alcohol/resin base mixture are combined with an aqueous phosphoric acid diluent solution. The acid roughens the metal surface and initiates cross-linking of the resin to form a pigment-filled polymeric film. The chromate pigment may also be dispersed in other carriers that are not as harsh as the wash primer. However, if a corrosion-inhibiting pigment can survive the harsh conditions of acid diluent, then it can usually be successfully incorporated within other paint, polymeric, or barrier film systems for corrosion inhibition.
An important use of chromate pigments is in coil coating formulations for steel, zinc-coated steel, or aluminum sheet stock. Coil coatings can represent a challenging application environment for pigments in that cure temperatures for these paints can exceed 100 C. Corrosion-inhibiting pigments for these applications must exhibit both throwing power to inhibit corrosion and be thermally stable at elevated temperatures when incorporated into the paint.
Lead chromate pigments of various compositions are mostly used as colorants, although they are also used for corrosion protection, albeit often for different applications than zinc or strontium chromates. This is necessitated by the much lower solubility of lead chromate in comparison to these other chromates. The dark yellow lead chromate (PbCrO4, C.I. Pigment Yellow 34) exhibits a solubility of approximately 10−7 moles/liter Cr+6, whereas the darker basic lead chromate (PbCrO4.PbO, C.I. Pigment Orange 21) exhibits slightly lower solubility in water. In addition, lead chromate can be combined with other lead compounds to alter the spectral or solubility characteristics. For example, the lemon yellow lead silicochromate (PbCrO4.PbSiO3) and reddish lead molybdate-chromate (PbCrO4.PbMoO4, C.I. Pigment Red 104) are representative examples. [Note: This same situation exists for zinc chromate pigments such as ZnCrO4, ZnCrO4.Zn(OH)2, and ZnCrO4(OH).Zn(OH)2, wherein increased basicity leads to lowering of the solubility.]
While lead chromate can be used in some of the same coating vehicles as zinc or strontium chromate (i.e., alkyd-type primers), the much lower solubility of lead chromate permits its use in binder systems for which zinc or strontium chromate would be unsuitable. For example, lead chromate is often used in oil-based paints (i.e., linseed oil) wherein greater than 75% by weight of the paint system is pigment. The mechanistic action of the divalent lead ion present in the lead chromate pigments is discussed below.
Pigments that contain tetravalent lead (PbIV) are also used extensively in coatings for metal protection, especially for steels and cast irons. The most notable PbIV pigment is red lead (Pb3O4, C.I. Pigment Red 105). Red lead is compositionally equivalent to lead plumbate [(Pb2+)2(PbO44−)], wherein only one of the constituent lead atoms is in the tetravalent oxidation state, while the other two are present as the more common divalent species. The PbVI solubility for this compound is a remarkably low 10−17 moles/liter Pb+4 (Glasstone, J. Chem. Soc. 121: 1456-69, 1922), implying an extremely low release rate of PbIV. Red lead is used primarily in alkyd-type primers as well as oil-based paints such as those containing linseed oil.
Calcium plumbate (Ca2PbO4, Caldiox™) is another representative example of a corrosion-inhibiting pigment containing PbIV. Due to its higher solubility in comparison to red lead, calcium plumbate is most frequently used for applications where a quicker release of oxidizing ions is necessary for protection, such as on galvanized steel. Calcium plumbate is most frequently used in alkyd-type or acrylic primers.
The use of hexavalent chromium or tetravalent lead pigments allows a broad range of service conditions to be addressed by tailoring the solubility of the pigment to the application needs. Very low solubility pigments, such as red lead, employ other corrosion-inhibiting mechanisms in addition to oxidative passivation. The divalent lead ions present in red lead (and even lead chromate) have been observed to form “lead soaps” after long storage when the pigment is used in conjunction with oil paints, especially those comprised of linseed oil. “Lead soaps” are reaction products between the alkaline divalent lead ions and the long-chain carboxylic acids present (i.e., oleates, linoleates, linolenates, palmitates, and stearates). This indicates that even though these pigments exhibit low solubility in water, the constituent ions are still chemically active, but less than their more soluble cousins. The divalent lead ions form insoluble compounds with corrosive anions such as sulfate, and low-solubility compounds with other corrosive ions such as chloride, thereby effectively removing them from the corroding system. For the heavier, more insoluble lead pigments such as red lead and basic lead chromate, the inhibiting action is due to both the oxidative tetravalent lead or hexavalent chromium ions, and the ability of the divalent lead ions to ‘capture’ (insolubilize) corrosive anions such as sulfate.
Significant efforts have been made in government and industry to replace CrVI and PbIV with other metals for corrosion-inhibiting applications due to toxicity, environmental, and regulatory considerations. An effective replacement for hexavalent chromate or tetravalent lead pigment needs to have throwing power for self-healing coating breeches. Throwing power is the ability of a highly oxidized ion, such as hexavalent chromium or tetravalent lead, to oxidize and passivate the exposed bare metal in a small scratch or flaw.
A number of materials have been introduced as corrosion-inhibiting replacement pigments for tetravalent lead- or hexavalent chromium-based compounds. Commercially available corrosion inhibiting pigments including compounds such as molybdates, phosphates, silicates, cyanamides, and borates that have no inherent oxidizing character have been used as alternatives to chromate pigments. Coatings that contain these materials can effectively inhibit corrosion as barrier films until the coating is breached, as by a scratch or other flaw. Films or coatings that do not contain oxidizing species can actually enhance corrosion on a surface after failure due to the effects of crevice corrosion.
Manganese is one non-toxic, non-regulated metal which has been considered as a chromium or lead replacement. Manganese (like chromium or lead) exhibits more than one oxidation state (Mn+2, Mn+3, and Mn+4). In addition, the oxidation-reduction potential is comparable to that of CrVI or PbIV in acidic solutions. For example, in acid solution:
Mn+3 + e−   Mn+2+1.49 VMn+4 + e−   Mn+3+1.65 VCr+6 + 3e−   Cr+3+1.36 VPb+4 + 2e−   Pb+2+1.70 V
The MnIV and MnIII ions are very good oxidizing species with oxidation-reduction potentials of +1.65, V and +1.49, V (at pH 0), respectively. The hydroxyl and oxygen liberated from water when MnIV or MnIII is reduced will oxidize nearby bare metal. This results in a passivated metal surface if sufficient oxygen is released. The potential required to reduce tetravalent or trivalent manganese to divalent manganese is only 0.29 or 0.13 volts greater than that needed to add three electrons to reduce CrVI to trivalent chromium (CrIII). Although neither MnIV or MnIII match PbIV in terms of redox potential, neither is significantly lower and so comparable passivation of metal is achieved. MnII is formed during corrosion inhibition by the oxidation of base metal in the presence of MnIV or MnIII and water. MnII is similar to CrIII in that neither is particularly effective as redox-based corrosion inhibitors.
A number of pigments using manganese have been reported in the literature over the years, but none approach the general performance or utility of PbIV- or CrVI-based pigments. For example, Manganese Violet is a colorant composed of MnII pyrophosphate with no oxidizing characteristics.
A number of compounds have been described as corrosion-inhibiting agents, including organic mercapto and thio compounds, cyclic tetraaza compounds, aminophosphonic acid, and triazinedithiols and triazinetrithiols. Other compounds have been described as corrosion inhibiting when complexed with manganese, typically in the divalent charge state. Among these compounds are porphyrin derivatives, tetraaza organic compounds, phosphoric and phosphonic acids, naphthenates, amidosulfonic acids, and amino acids. However, the pigments formed from these compounds provide only limited corrosion protection and do not approach the benefit derived from the use of hexavalent chromium.
In addition, the formation of manganese-containing pigments in which the manganese is complexed with ligands such as hydrazones, —O bidentates, azomethines, phthalocyanines, azo and disazo complexes, N—S bidentates, oximes, tetraaza complexes, porphyrins, 1,2-dithiolates, and semicarbazones, has also been described. However, these compounds do not use tetravalent or trivalent manganese and are not used for anticorrosive applications.
U.S. Pat. No. 6,416,868 to Sullivan, et al. describes the use of alkaline earth-manganese oxides as pigments, wherein the alkaline earth is selected from Mg, Ca, Sr, and Ba. The pigments are described as being colorants and as resisting heat build-up due to infrared reflectance. Corrosion inhibiting characteristics of these compositions are not discussed or claimed. Additional manganese-containing colorants, frits, and ceramic pigments with no discussed or claimed anticorrosive properties are identified in U.S. Pat. Nos. 4,159,207 to Nuss, 3,832,205 to Lowery, and 5,254,162 to Speer, et al., and German Patent Nos. DE 41 31 548 and DE 40 02 564, both to Speer, et al.
U.S. Pat. No. 4,469,521 to Salensky describes a corrosion-inhibiting pigment formed by sintering manganomanganic oxide (Mn3O4) with oxides of calcium, zinc, barium, magnesium, or strontium.
U.S. Pat. No. 4,388,118 to Eppler describes a corrosion-inhibiting black pigment formed by calcining calcium or strontium oxides or carbonates with manganese oxides. The formed calcium manganese oxide or strontium manganese oxide pigments have very low oil absorption and high tint strength.
German Patents Nos. 26 42 049, 28 15 306, and 26 25 401 to Hund, et al. describe the use of calcined calcium/iron/aluminum/manganese oxides as corrosion-inhibiting pigments.
Manganese oxides themselves have been described as being constituents in corrosion-inhibiting formulations. For example, U.S. Pat. Nos. 4,417,007 and 4,417,008 to Salensky, et al. as well as Belgian Patent Nos. BE 893,677 to Chopra, et al. and BE 893,676 to Salensky, et al. describe manganomanganic oxide (Mn3O4) as a constituent in a solvent-based paint formulation comprised of resin binders and additional extenders, fillers, and solvents. Manganese dioxide (MnO2) is also described as an anticorrosive constituent within formulations in French Patent No. 2,348,257 to Zatmann and German Patent No. DE 39 35 478 to Ortlepp.
U.S. Pat. No. 4,788,411 to Skinner describes a paint composition described to prevent corrosion of welds. This composition is described as being composed primarily of zinc dust, with manganomanganic oxide (Mn3O4) and organic binder included. The zinc pigment is described as being especially important for corrosion protection.
To date, no truly effective replacements have been developed for pigments based on CrVI or PbIV. Accordingly, the need remains for improved corrosion-protective pigments composed of currently unregulated and/or nontoxic materials which have an effectiveness, ease of application, and performance comparable to current CrVI or PbIV pigment formulations, and for methods of making and using the same.